1. Field of the Invention
This invention relates to a hydraulic modulator for an anti-lock brake and traction control system for a vehicle. More particularly, the invention relates to such a system which is capable of imparting a suitable braking force to wheels at the time of starting the vehicle to prevent drive slippage of the wheels, and which is also capable of controlling the braking force, applied to the wheels, to an optimum level at the time of braking of the vehicle to prevent locking of the wheels.
2. Discussion of the Related Art
Recently, anti-lock brake systems have been developed for avoiding locking of the wheels at the time of braking of the wheels, as well as traction control systems which prevent excessive slippage of drive wheels at the time of abrupt start or acceleration of the vehicle (this is called "traction control") so as to improve driving operation, thereby enhancing the starting and accelerating abilities and driving stability. One known hydraulic modulator for such an anti-lock brake and traction control system for a vehicle is described in U.S. Pat. No. 4,509,802.
The general construction of the hydraulic modulator disclosed in U.S. Pat. No. 4,509,802 will now be described with reference to FIG. 6. As shown in this Figure, two liquid pressure-generating chambers are formed within a cylinder body of a tandem master cylinder 102, and a reservoir 101 is integrated with the body 102. Lines 103 and 104 are connected respectively to the two liquid pressure chambers, and a liquid pressure can be produced in the lines 103 and 104 depressing a brake pedal 100.
The line 104, connected to one of the liquid pressure chambers, is connected via a cut valve 109 and brake pressure control valves 105 to brake calipers 106 for the drive wheels. As with the drive wheel side, the line 103 is connected via brake pressure control valves to brake calipers of the driven wheels. The cut valve 109 is a two-position (D and E) switch valve, and the brake pressure control valve 105 is a three-port (A, B and C) three-position switch valve.
Lines 120, connected to the brake pressure control valves 105 of the drive wheel side, are connected to a reservoir 108, and the reservoir 108 is connected via a check valve 113 to a suction port of a hydraulic pump 107 driven by a motor. A discharge port of the hydraulic pump 107 is connected to the cut valve 109 via a check valve 113 and a throttle. As shown in the drawings, each check valve 113 comprises a ball and a spring.
The cut valve 109 is connected to a pressure tank 111, and is also connected to a reservoir tank 101 via a relief valve 110. At the time of the traction control, the pressure tank 111 supplies a brake liquid to the brake calipers 106 via the cut valve 109, and has a capacity sufficient to accumulate an amount of brake liquid required for the traction control. With respect to the accumulation in this pressure tank 111, the cut valve 109 is switched to the position E during parking of the vehicle, and when the brake pedal is depressed, the liquid pressure produced in the tandem master cylinder 102 is accumulated in the pressure tank 111 via the check valve 114 and the cut valve 109. Alternatively, during parking of the vehicle, the cut valve 109 and a switch valve 112 are switched to the position E and the position F, respectively, and the hydraulic pump 107 is operated, so that the pressure is accumulated via the reservoir tank 101, the switch valve 112, the hydraulic pump 107 and the cut valve 109, utilizing the discharge pressure of the hydraulic pump 107. The relief valve 110 serves to achieve a constant pressure in the circuit, and also serves as a safety mechanism. For example, in the case where the pressure from the hydraulic pump 107 is to be accumulated in the pressure tank 111, when the pressure discharged from the hydraulic pump 107 becomes unduly high, the brake liquid is transmitted back from the relief valve 110 to the reservoir tank 101 via the cut valve 109 disposed in the E position, thereby preventing damage to the equipment.
The operation of the hydraulic brake device of the above construction is as follows. Referring first to the slip control of the drive wheels, for example, at the time of the start of the vehicle, if the vehicle is started abruptly, so that the drive wheels are subjected to slippage of a level greater than a predetermined value because of an excessive engine torque, an electronic control device detects this condition through a wheel speed sensor. The cut valve 109 is then switched to the position E, and brake liquid of a predetermined pressure is supplied from the pressure tank 111 to the drive wheel brake calipers 106 via the brake pressure control valves 105. As a result, the drive wheels are braked, thereby reducing slippage of the wheels.
During this slip control, the brake control valve 105 can take the positions A, B and C under the control of the electronic control device so as to repeatedly increase, hold and decrease the braking force, thereby controlling the drive slip to the optimum condition. Then, when the drive slip becomes almost zero, the electronic control device detects this, and stops the driving of the hydraulic pump. In this manner, the slip of the wheels at the time of the start of the vehicle is controlled, so that the vehicle can be started smoothly. Thereafter, the brake liquid is again accumulated in the accumulator 111 in the above-mentioned manner.
When the brake pedal 100 is abruptly pressed down during the travel of the vehicle to apply an abrupt braking, the wheels tend to lock. Reference is now made to a brake control for such a condition. When the wheels tend to be locked, the electronic control device judges that the brake pressure should first be maintained at a constant level, and sends a signal to the brake pressure control valve 105 to switch this brake pressure control valve to the position B, thereby holding the wheel braking force at the constant level. When the electronic control device judges that it is not sufficient to merely hold the braking pressure at the constant level, and that the braking pressure should be decreased, the brake pressure control valve 105 is switched to the position C, and the pressure liquid of the brake caliper for each wheel is discharged to the reservoir 108 via the brake pressure control valve 105 and the line 120, so that the braking is reduced. The brake liquid, discharged to the reservoir 108, is immediately drawn into the hydraulic pump 107, and the brake liquid discharged from the hydraulic pump 107 is returned to the lines 103 and 104, and is used to again increase the pressure through the brake pressure control valve 105 disposed in the position A. Otherwise, this brake liquid is returned to the tandem master cylinder 102. Consequently, the tendency for each wheel to be locked is overcome, so that the vehicle can be decelerated or stopped in a stable manner.
In the above hydraulic modulator, however, it is necessary to accumulate liquid pressure in the pressure tank 111 after traction control or when this liquid pressure decreases for some reason. This must be done in preparation for the next traction control. Therefore, in order to accumulate pressure in the pressure tank 111, the accumulation has been done by depressing the brake pedal during the parking of the vehicle, or by suitably switching the cut valve 109 during the operation of the anti-lock brake system so as to utilize the pressure discharged from the hydraulic pump 107. Therefore, the switching control of the cut valve 109, the control of the hydraulic pump 107 and so on must be effected by the electronic control device. This has resulted in a complicated and expensive device. In order to overcome such a drawback, a device has been proposed in which a brake liquid pressure, produced in a master cylinder during a normal braking operation, is accumulated in an accumulator via a one-way valve. In this type of device, however, when the brake pedal is depressed many times, the brake liquid pressure accumulated in the accumulator becomes unduly high. As a result, the high liquid pressure acts on seal portions of a traction control hydraulic circuit for a long period of time, which results in premature deterioration of the seal portion. In this type of device, when the brake liquid is supplied to the accumulator, the pedal stroke becomes greater than usual, which undesirably causes the driver to sense a difference.